1. Field of the Invention
This invention relates to a method for the treatment of waste water. Particularly, it relates to a method for effecting wet oxidation of waste water containing a substance of chemical oxygen demand (hereinafter referred to as "COD component") in the presence of a catalyst. More particularly, it relates to a method for effectively detoxifying waste water containing a COD component, i.e. a harmful oxidizable organic or inorganic substance, by subjecting the waste water to catalytically wet oxidation in the presence of molecular oxygen thereby converting the harmful substance into such harmless compounds as carbon dioxide, water, and nitrogen.
2. Description of the Prior Art
Among the conventional means available for the treatment of waste water, there are counted a biochemical method called an activated sludge method and a wet oxidation method called a Zimmerman method.
As widely known, the activated sludge method consumes much time in the decomposition of organic matter and, moreover, requires the waste water to be diluted to a concentration suitable for the of algae and bacteria and, therefore, has a disadvantage that the ground area for installation of facilities for the activated sludge treatment is very large. Further in recent years, particularly in the urban districts, the disposal of grown excess sludge has been entailing a huge expense. The Zimmerman method consists in effecting oxidative decomposition of organic matter contained in a high concentration in an aqueous solution by introducing air under a pressure in the range of 16 to 200 atmospheres at a temperature in the range of 200.degree. to 370.degree. C. into the aqueous solution. This method requires to use a large reactor because the reaction proceeds slowly and the decomposition consumes much time. Further, the reactor itself requires the material thereof to possess high durability. Thus, this method is not economically advantageous because of high cost of equipment and high cost of operation. In connection with this method, it has been proposed to use various oxidative catalysts aimed at accelerating the reaction velocity.
Among the catalysts heretofore used popularly in the catalytically wet oxidation method, there are counted compounds of such noble metals as palladium and platinum (Japanese Patent Laid-Open No. SHO 49(1974)-44,556) and compounds of such heavy metals as cobalt and iron (Japanese Patent Laid-Open No. SHO 49(1974)-94,157). They are catalysts which have the compounds deposited on spherical or cylinderical carriers of alumina, silica-alumina, silica gel, and activated carbon. More often than not, in the catalystically wet oxidation of waste water, these catalysts are put to use in the reaction at a pH value of not less than 9. In our experiment, the catalysts, in the course of their protracted use, have been found to suffer from loss of strength and disintegration of individual particles and, in an extreme case, entail dissolution of their carrier.
In an effort to solve this problem, there have been recently proposed methods for reinforcing the catalysts by using titania or zirconia as a carrier therefor (Japanese Patent Laid-Open Nos. SHO 58(1983)-64,188 and SHO 59(1984)-19757). To be specific, these inventions disclose catalysts which have compounds of such noble metals as palladium and platinum or compounds of such heavy metals as iron and cobalt deposited on spherical or cylindrical carrier particles of titania or zirconia. By experiment, the carriers are certainly found to possess greater strength than the conventional carriers. These catalysts, however, are invariably in a particulate form. Moreover, they are not fully satisfactory in terms of catalytic activity and durability.
Incidentally, when the waste water is subjected to wet oxidation, there inevitably arises the necessity of disposing of a large volume of water. As regards the manner of reaction, therefore, the method of using a fixed bed in a system designed for flow of waste water is frequently adopted. Moreover, numerous samples of waste water contain solid substances. If, in any of these cases, a given catalyst is in a particulate form, since the flow of the waste water induces heavy loss of pressure, the waste water cannot be treated at a high linear velocity and the current of this waste water must be given a large cross section. In the treatment of waste water entraining a solid substance, since the solid substance clogs the fixed bed of catalyst even to an extent of increasing the resistance which the fixed bed of catalyst offers against the flow of waste water, this treatment entails a disadvantage that the running cost is increased and the apparatus for treatment cannot be operated continuously for a long time. In the treatment of waste water by the catalytically wet oxidation method using such particulate shaped catalyst, since the reaction is carried out at an elevated temperature under a high pressure and the ground area occupied by the reactor must be proportionately large, the cost of equipment is huge. The high cost of equipment constitutes a critical problem.
As regards the life of catalyst, the particulate catalyst has a disadvantage that by mutual contact of the particles, the catalyst is comminuted by friction and disintergration. For the purpose of diminishing the loss of pressure due to the catalyst bed, there has been proposed a fluidized-bed method which comprises fluidizing a catalyst in a powdery form in a current of waste water. This method, however, has not yet been adopted for actual use because the catalyst by nature is diluted and the reactor used for the treatment, therefore, must possess a huge capacity and the separation of the catalyst form the treated waste water is very difficult.
There also are methods which effect oxidative decomposition of organic matter in waste water at normal room temperature under normal atmospheric pressure by using ozone or hydrogen peroxide as an oxidizing agent. Japanese Patent Laid-Open No. SHO 58(1983)-55,088, for example, discloses a method which effects oxidative decomposition of such organic substances as fumic acid contained in waste water by treating this waste water with ozone and hydrogen peroxide at 20.degree. C. under normal atmospheric pressure in the absence of a catalyst. Japanese Patent Publication No. SHO 58(1983)-37,039 discloses a method which effects oxidative decomposition of an aromatic ring-containing organic compound contained in waste water by adding a surfactant to the waste water, further adding thereto at least one member selected from the group consisting of transient metal compounds and alkaline earth metal compounds, and then exposing the resultant mixture to ozone at normal room temperature under normal atmospheric pressure. Since the former method effects the treatment in the absence of a catalyst, it is incapable of effectively treating sparingly oxidizable substances suspended in waste water. Since the latter method uses the metal ion such as of transient metal or alkaline earth metal as a catalyst, it is required to recover the metal ion contained in the treated waste water before this treated waste water is released into a nearby body of running water. It has a disadvantage that it inevitably requires an extra step of aftertreatment. Further, since both these methods require the treatments for waste water to be carried out at normal room temperature under normal atmospheric pressure, they have a disadvantage that they call for supply of a large volume of expensive ozone, their reactions proceed at a low rate, their ratios of decomposition of organic matter are low, and the treated waste water require a treatment for detoxification because the unaltered ozone leaks in the treated water.
An object of the present invention, therefore, is to provide a method for efficient and long continuous treatment of waste water.
Another object of this invention is to provide a method for enabling the treatment of waste water to be carried out efficiently in a high linear velocity.
Yet another object of this invention is to provide a method for enabling waste water containing solid substances to be treated stably at a high linear velocity continuously for a long time.